1. Field of the Invention
This invention relates to a control circuit for a brushless electric motor and more particularly to a novel improvement in which the preceding signal is maintained until the dead zone is elapsed and the signal for switching the direction of rotation is supplied to the gate drive circuit only in synchronism with the pulse generated by the pulse width modulating circuit (PWM circuit).
2. Prior Art
There are a variety of control circuits for brushless electric motors of the above type which are so far known and employed in the art. Typical of these is a system as shown in FIG. 1 and tentatively produced by the present assignee for use within the assignee's company, although the title of the literature showing the structure is not given herein.
Now referring to FIG. 1, the numeral 1 denotes a current amplifier adapted for amplifying the electric current from a current input terminal 2. The signals 1a from the current amplifier 1 are input to a pulse width modulating circuit or PWM circuit 3 and to a comparator 4.
The pulse signals 3a from the PWM circuit 3 are applied to a gate drive circuit 5, while signals 4a for switching the direction of rotation are supplied from the comparator 4 to the gate drive circuit 5.
To the output side of the gate drive circuit 5, there is connected an inverter circuit 6 consisting of six switching transistors Q.sub.1 to Q.sub.6. An electrical source 7 is connected to these switching transistors Q.sub.1 to Q.sub.6, while a diode 8 is connected across the emitter and the collector of each of the switching transistors Q.sub.1 to Q.sub.6.
The inverter circuit 6 is so arranged and designed that the switching transistors Q.sub.1 and Q.sub.2 control a U-phase drive coil 9, the switching transistors Q.sub.3 and Q.sub.4 control a V-phase drive coil 10 and the switching transistors Q.sub.5 and Q.sub.6 control a W-phase drive coil 11.
These U-, V- and W-phase drive coils 9, 10 and 11 constitute a stator winding 14 applied to a stator iron core 13 of a brushless electric motor shown in FIG. 4.
A rotor shaft 20 made fast with a rotor 19 including a rotor iron core 17 and a permanent magnet 18 is rotatably mounted between bearings 16, 16 of a frame 15 of the burhsless electric motor 12. The extreme end of the rotor shaft 20 is made fast with a Hall rotor 22 including a permanent magnet 21. Within the motor frame 15, there is provided a Hall assembly 24 having a Hall element 23 operatively associated with the permanent magnet 21. The Hall rotor 22 and the Hall assembly 24 constitute a rectification signal generator 25. An encoder 12a is provided on the outer side of the generator 25.
The rectified signals obtained from the generator 25 are applied to the gate drive circuit 5.
The U-, V- and W-phase drive coils 9-11 are respectively provided with current sensors 9a, 10a and 11a connected in turn to a current sensor circuit 26, the output of which is supplied to the input terminal 2.
The above-described prior art brushless electric motor operates as follows:
When the rotor 19 is to be rotated in any one direction, pulse signals 3a from PWM circuit 3 are applied to the gate drive circuit 5 so that the upper switching transistors Q.sub.1, Q.sub.3 and Q.sub.5 or the lower switching transistors Q.sub.2, Q.sub.4 and Q.sub.6 of the inverter circuit 6 are turned on and off, while the lower transistors Q.sub.2, Q.sub.4 and Q.sub.6 or the upper transistors Q.sub.1, Q.sub.3 and Q.sub.5 are turned on and off by the rectified signals 25a for sequentially switching the U-, V- and W-phase drive coils 9, 10 and 11 to effect rotation of the rotor 19.
When the signals for switching the direction of rotation 4a are to be produced, these signals are constituted by the high or low level signals derived from the output signals from the comparator 4 to realize the desired rotational direction of the motor.
Turning now to the waveform diagram of FIG. 3, the operation of switching the rotational direction is explained more specifically.
It is now assumed that, with the output waveform 1a from the current amplifier 1 being as shown at A before time t.sub.1, transistor Q.sub.1 is turned on, while the transistor Q.sub.4 is turned or and off by the pulse signals 3a of the PWM circuit 3.
In such case, the output waveform 1a comes into the dead zone of the PWM circuit 3, such that the transistors Q.sub.1 is turned on but the transistor Q.sub.4 is turned off.
Then, at time t.sub.2, the signal for switching the direction of rotation 4a is produced from the comparator 4 such that the transistor Q.sub.1 is turned off while the transistor Q.sub.3 is turned on. At this time, the transistors Q.sub.2 and Q.sub.4 remain in the turned-off state.
Then, at time t.sub.3, the output waveform 1a from the current amplifier 1 comes out of the dead zone of the PWM circuit 3, so that the turned-on state of the pulse is initiated.
The above described prior-art brushless electric motor suffers from the following problems. In the operating mode shown in FIG. 3, and at time t.sub.2, the current flows through one of the coils 9 to 11 through the diode 8 of the inverter circuit 6 under the counter electromotive force produced by the motor.
Since the time interval t.sub.1 -t.sub.2 corresponds to the dead zone of the PWM circuit 3, the current does not flow through any one of coils 9-11. However, the current flowing at t.sub.2 as described above is sensed by the current sensor circuit 26 so that sensor signals 26a are produced.
After time t.sub.2, the output waveform 1a of the current amplifier 1 shifts towards the positive side. As the operating point of the comparator 4 is exceeded, the signal for switching the rotational direction 4a again goes to the low level between time t.sub.2 and time t.sub.3. Under such conditions, at time t.sub.3, although the drive current is supposed to flow by the operation of transistors Q.sub.3 and Q.sub.2 it flows instead by operation of transistors Q.sub.1 and Q.sub.4 so that the rotor 19 tries to rotate in the opposite direction. However, because reverse current is caused to flow under the instantaneous operation of the servo, regular operation is resumed after several repetitions of the above described operations.
During that time, the signal for switching the direction of rotation 4a changes, as indicated by the dotted lines in FIG. 3, to create great deal of noise and vibrations.
In this manner, at the time of switching the direction of rotation of the motor, unnecessary mechanical operation occurs, with the result that the user may be disagreeably bothered, while the service life of the motor is also affected adversely.